Prior to this invention, an electrically operated control system has been used for a hydraulically operated tractor hitch assembly of the type including a rockshaft operated by a main cylinder which is controlled through a main valve operated by a pilot cylinder which is controlled, in turn, through a solenoid operated pilot valve. In that system, the pilot valve is connected to the output of an amplifier circuit responsive to an error signal developed as a function of a comparison between the position of a manually operable control and an operational condition of the hitch assembly such as its vertical position. The amplifier circuit is also responsive to a feedback signal developed in response to movement of a spool or control element of the main valve and applied in opposition to the error signal.
With that system, disadvantages of manual operation of the main valve are overcome and highly satisfactory results can be achieved. However, there are problems in obtaining a fast response to movements of the manually operable control and in obtaining accurate control without running into problems in connection with oscillations and instabilities in the operation of the system.
It is also noted that various other types of electrically controlled servo systems have been used in many applications but for a number of reasons, such systems have not been entirely suitable for control of a tractor hitch assembly.